Constant Current Dummy Load

There are always these few projects you look at which seem simple on the surface but ones you start getting into the nitty-gritty of the project it ends up teaching you a lot.

This project is no exception. Let us first understand the use of the project.

Snapseed(1)

Say you have bought a new battery for your project and you want to discharge it to see its performance or say you just built a power supply and now you want to load the supply and see how it performs under various load conditions, your primary option is to use multiple high wattage resistors in combination to get the right amount of load and also you have to keep swapping the resistances to check for various load conditions but there is a better way. You can use an electronics load, where you can just set the amount you current you want to draw and it will load your circuit/battery for that current (also you can make it draw maximum current at a certain voltage or set a constant wattage but we are sticking to the basics here).

I first saw this circuit made by Dave Jones on EEVBlog and he made the circuit look very simple which it is but there are lots of details you can get into later on when you start building the circuit with the parts you have laying around.

This is not an explanatory blog for the circuit but if you want that Dave and many other YouTubers have done a fabulous job of that. I just made this circuit early on during my engineering for one of my other projects as a testing instrument and early this year I put the whole project in a case and now have decided to make the PCB for it to add active cooling to it. So I thought, might as well document it and share the files so others can make it for themselves. I will be using mostly off-the-shelf parts so the project can be replicated easily. There are a few changes I have made to the original circuit to make the circuit more useful which I will explain below.


Please find the Schematic PDF file and other design files on my GitHub page.


Let start by taking an overview of the schematic and understand component choices I made along the way.

The device is called Constant Current Dummy Load which has a fully analog design and no digital logic in it. This device was designed to test a few low power DC-DC Converters design so I didn’t need any large current load, so I decided to have a maximum 2Amp load with 1Amp & 2Amp range switch so that I can get the range of 2A but also have a fine range selection for lower currents. I am using a 10 Turn pot for setting the current.

If you see the schematic we have mainly 3 sections here (actually there are 4 but the DC fan circuit which is shown in the schematic PDF is not implemented in my current design so will talk about it in a later post)

  • ON-OFF & Range Switch
  • Main Control Loop
  • Voltage Set & Read Switch

ON-OFF & Range Switch

voltage_selection

As the name suggests its the on-off switch for the device and it is also used to switch between two ranges. It is a basic voltage divider circuit with Dual Pole Dual Throw rocker switch with a centre off. I am using a 9v battery to power this device.

Component Selection

There are no specific values to be used as such just use the resistors which will get you the ranges required for the circuit and also keep the resistance high to keep current consumption low. This circuit is not ideal because the voltage range will change as the battery voltage drops over time so just add in a voltage regulator to have a constant voltage and use that for the op-amp input.

Also, you will be needing a multi-turn potentiometer as you can do finer adjustments with it.

 

Main Control Loop

main_loop 

This is the main circuit which is quite simple to understand. I have first op-amp ‘A’ from the LM324 quad op-amp as a voltage buffer with its output going to the next op-amp which is controlling the MOSFET IRFZ44N. Mosfet is used to control the current going through a 1ohm resistor which is loading the “PowerInn” connector which is the input supply to be loaded. The second opamp is given the feedback of the voltage drop across the 1ohm resistor. If you know the three golden rules of the opamp, you will know that opamp will do everything in its power to keep the voltage across its inverting and non-inverting pin equal. As the voltage at its non-inverting pin is the voltage we set using the pot, say 100mV the output of the opamp goes high and turns the Mosfet ON which flows current through the 1omh resistor as soon as the voltage drop across the resistor is 100mV which is fed back to the opamp’s inverting pin the opamp pulls the output low and the Mosfet turns off, this goes on and we get a 100mV across the resistor now as the resistor is 1ohm resistor, using ohms law we get 100mA of current is flowing through the resistor. So we can get any voltage across the load resistor which will translate into the current flowing through the resistor we get a Constant Current Sink. If you may have noticed there are few passives in the control loop which I have not mentioned which are used to stabilize the control loop as the op-amp is driving a capacitive load, also there is an inductive component of the leads to be used to connect the power supply to the device which also can make the opamp control loop unstable.

Component Selection

LM324 – I selected this opamp because I had it laying in my parts bin but ideally you will want an opamp with low input offset which also can go rail to rail at its output as having higher voltage at the gate of Mosfet is better.

IRFZ44N – This MOSFET was also selected because I had it in my parts bin, but again ideally you would want a MOSFET with a low threshold voltage or turn on voltage, also mind the gate capacitance it should be low, Lower ON Resistance of the opamp is also a plus and choose a package which can handle the current you need to load.

1ohm Resistance – I am using a single 1ohm resistance of 25W, 5% tolerance, but if possible use a more accurate resistance and do the thermal calculations according to your current requirements.

Passives in the control loop – The values of the passives in the control loop is something you need to determine and will change according to your other component selection. I used LTSpice to model the circuit and derive the ideal values for my selected components, below is a snippet of the circuit model and the simulation output.

ltspice

 

Voltage Set&Read Switch 

set_read_amp

I added in this circuit to the original circuit as there was no way to see the set current, so I added in a toggle switch with an opamp as a buffer which goes to a banana plug connector shown as “Signal O/P” to connect a multimeter to see the reading, this can also be easily connected to a voltmeter on the device itself just take care of the scaling as now I have used 1ohm resistor so I get 1V out for 1A but if you used lower resistors like 0.1ohm then would need to add gain to the output opamp.

 

Finally following are the photos of the device in an enclosure and also few photos of the insides of the enclosure.  

 

Now I am in process of designing the PCB for it as the current circuit is made on prototyping board and is very messy in terms of its wiring. Also, I will be adding a DC fan to the device to get a better cooling performance which I will be updating in my next blog post.

See you then.

 

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